Add support for the More comments type through the reddit 'api/morechildren' API. Currently returns a mixed-type RKComments/RKMoreComments object array. Changes in this commit include:
• Create new RKMoreComments class.
• Create new RKMoreComments objects in RKComment:repliesJSONTransformer when processing JSON that contains data for both RKComments and RKMoreComments.
• Update RKComment:JSONKeyPathsByPropertyKey to properly set a complete set of RKComment object properties from JSON data.
• Update RKClient+Comments API to handle a maximum 'limit' parameter (since the concept of pagination isn't supported for comments) for the initial comments query.
• Add a new API in RKClient+Comments: moreComments:forLink:limit:completion.
• Add a new API in RKClient+Requests: postMoreCommentsListingTaskWithPath:parameters:completion and accompanying internal helper method objectsFromMoreCommentsListingResponse.
• Update RKObjectBuilder.m to return the RKMoreComments class when detected in the objectKind JSON string.